Metal-air cells typically include a metal anode, an air cathode, and a separator all disposed and supported in some sort of container. The metal anode usually comprises a fine-grained metal powder, such as zinc, aluminum, or magnesium, which is blended together with an aqueous electrolyte, such as potassium hydroxide, and a gelling agent into a paste. The separator is a porous material that allows the passage of electrolyte between the cathode and anode, but prevents direct electrical contact therebetween and short circuiting of the cell.
The air cathode is a catalytic structure designed to facilitate the reduction of oxygen. Typically, it is composed of active carbon, a binder, and a catalyst which, together with a metal current collector, are formed into a thin sheet. The air cathode also commonly incorporates a hydrophobic polymer, such as polytetrafluoroethylene or polystyrene, directly into the cathode sheet and sometimes also as a coextensive film. The hydrophobic polymer prevents electrolyte from flooding the cathode or passing through it and leaking from the cell. The container includes oxygen access openings, diffusion chambers and the like which are designed to allow sufficient oxygen to reach all parts of the air cathode.
Metal-air cells have high specific energies. In fact, zinc-air cells have the highest specific energy, up to 450 Wh/kg, of all aqueous primary systems, and high energy per unit volume as well. The components of zinc-air cells also are relatively benign.
Because of their high energy density, button cells incorporating zinc-air chemistry currently are the most popular energy source for hearing aids. The much larger majority of electronic devices, however, has higher energy requirements requiring the use of larger (i.e., greater than one ampere hour capacity) cells or batteries. Despite the electrochemical advantages of metal-air and especially zinc-air systems, heavy duty and alkaline manganese systems continue to dominate the much larger world market for larger primary batteries.
Many portable electronic devices, such as portable computers, also place severe constraints on battery weight and volume. In such applications, prismatic cells would be preferable over button or cylindrical cells, which latter type of cells, in general, require more space to be allocated in the device than the cells themselves actually occupy. Prismatic zinc-air cells also can be much thinner than alkaline cells of equivalent capacity.
The cells disclosed in U.S. Pat. No. 5,328,778 to G. Woodruff et al. successfully scaled up and reconfigured metal-air cells from the traditional, relatively small button-style design, making them much more suitable for use in the full spectrum of battery powered electronic devices. There remain, however, various unmet performance demands which result when metal-air cells are used outside of their traditional hearing-aid applications.
That is, while oxygen must be able to diffuse into the cathode to discharge a metal-air cell, air circulation into the cell when it is not being used to power a device is generally deleterious. Any oxygen which reaches the anode will directly oxidize the zinc or other metal therein and diminish the electrochemical capacity of the cell which is available to power an electronic device. Moreover, gelled anodes are aqueous based, and moisture loss from the anode can facilitate direct oxidation of metal in the anode and by various other mechanisms reduce the discharge rates and capacity of the cell.
For this reason, new metal-air cells generally are packaged in high moisture barrier film, and the cells are not taken out of their package until they are to be used. Metal-air button cells for hearing aids also generally have very small air holes in their containers, to minimize moisture loss during their service life. Further, hearing aids typically are used for long periods with relatively short intervals of non-use. That is, hearing aids usually are worn all day and are turned off only at night when the user goes to bed. There simply is much less time in the service life of hearing aid cells during which the deleterious effects of unwanted air circulation can take their toll. Most other electronic devices are used intermittently, and there may be long intervals, between uses of the device, when the effects of oxygen access and moisture loss can build.
Accordingly, various battery and cell casings, housings, and the like have been proposed which incorporate a valve or equivalent means to shut off air circulation to a metal-air cell when it is not in service. More recently, U.S. Pat. No. 4,620,111, to W. McArthur et al. has disclosed a battery pack which uses button cells. A number of button cells are disposed in a housing. The housing has openings therein to allow air circulation into the housing and ultimately to the cells. The housing also includes a slide valve which controls the flow of air through the air circulation openings. The design of this housing, however, has significant shortcomings, and it is especially ill suited for use with prismatic cells.
While the slide valve is said to selectively "seal" the air circulation openings, inherently the seal is far from perfect. The part tolerances needed to allow easy movement of the slide valve between its open and shut positions also allow the valve, depending on the orientation of the housing, to shift away from the surface in which the openings are located. Under such conditions, air can flow under and around the slide valve. Moreover, even when the slide valve is resting over the openings, the seal is not perfect. The valve and housing are injection molded plastic parts and such parts inevitably have some warpage and other imperfections which create paths for air flow between the slide and the apertured surface in which the openings are located.
Such imperfect seals are undesirable when larger, prismatic cells are to be used, even if such valves are tolerable in the context of button cells. Button cells have very restricted gas access, and they are less sensitive to moisture loss. Prismatic cells, designed for higher power levels, have much less gas restriction, making them more sensitive to moisture loss.
Other "valved" metal-air batteries have been proposed which may offer more reliable sealing of a cell's oxygen access opening, such as disclosed in U.S. Pat. No. 4,177,327 to J. Mathews et al. and U.S. Pat. No. 4, 493,880 to J. Lund. In Mathews '327, the batteries include a flexible element having a synthetic rubber plug. The flexible element is biased such that in its vent-closed position the plug seats against a single oxygen access opening. Electrically powered actuator means are provided to move the flexible element and plug away from the opening to allow air into the battery.
Lund '880 discloses a sliding switch which, upon movement between the closed and open positions, selectively covers and uncovers a single oxygen access opening with an adhesive, flexible closure strip. The switch is provided with spring finger contacts which urge the slide downwardly to facilitate adhesion of the closure strip over the opening.
Both designs, however, are somewhat complicated and are not well suited to larger batteries which frequently have many oxygen access openings with a total cross-section which may be fairly large. For example, in larger batteries having larger slide valves, biasing the slide against the surface in which access openings are located would require either a large number of springs or a relatively heavy, stiff slide. Since the force is applied noncontinuously on the slide at discrete points, thinner, lightweight slides tend to flex away from the surface in areas distant from a spring, and flexing can diminish the overall effectiveness of the seal. Heavy, complicated valves, on the other hand, run counter to the recognized goal of producing simple, lightweight batteries.
An object of this invention, therefore, is to provide metal-air cells and batteries which are more suitable for use in powering electronic devices which are used intermittently over the service life of the battery.
It also is an object to provide prismatic metal air cells and batteries in which the gelled anode is less susceptible to oxygen attack and moisture loss resulting from unwanted air circulation.
A related and more specific object of the invention is to provide metal-air cells and batteries having a valved casing which allows air circulation when the cells are in use, but more effectively restricts the access of air to the cells during periods of non-use.
Another object is to provide such cells and batteries wherein the valve is more readily susceptible to use in relatively large cells having many oxygen access openings.
A further object of the invention is to provide such cells and batteries which may be manufactured easily and economically.
Yet another object is to provide such cells and batteries wherein all of the above-mentioned advantages are realized.
Those and other objects and advantages of the invention will be apparent to those skilled in the art upon reading the following detailed description and upon reference to the drawings.